Electrophoretic deposition of boron



United States Patent ELECTROPHORETIC DEPOSITION OF BORON Arthur E. Newkirk, Schenectady, N.Y., assignor to genleral Electric Company, a corporation of New Filed Dec. 27, 1966, Ser. No. 604,615 Int. Cl. C23b 13/00; B01k 5/02 U.S. c1. 204--181 1 5 Claims ABSTRACT OF THE DISCLOSURE Commercial boron powder from various sources is electrophoretically deposited at high rates employing one of the following liquids as the suspension medium: (a) a mixture containing isopropanol and nitromethane; (b) isopropanol, and (0) ethanol. The highest deposition rate was obtained with a mixture of isopropanol and nitromethane.

The basic principles of electrophoretic deposition (sometimes referred to as cataphoretic deposition) of boron are well known. In essence, particles of finely divided boron are suspended ina suitable liquid medium containing a pair of spaced electrodes. Upon applying a potential difference to the two electrodes, the particles of finely divided, suspended boron acquire a surface harge. by, whichn uehdparticl s ar rac ed totand d ad, ,7

posited on an electrode.

Commercial boron powders differ considerably from each other as to impurity content and-particlesize;Not

manufacture can be expected to perform in substantially the same way in the given medium.

The problem, therefore, faced by the purchaser of commercial boron powders, who wishes to eiectrophoretically deposit these powders, is that of correlating particular boron powders with specific liquid mediums in which they may be successfully deposited. The problem becomes particularly severe when the boron powder comes from an unknown source.

It is, therefore, an object of this invention to isolate a small group of liquid suspension mediums, at least one of which may be relied upon for use in the electrophoretic deposition of commercial boron powder regardless of the particular impurity content or the specific process of manufacture to yield a considerable increase in the rate of deposition as compared to the deposition rates from the electrophoretic liquid mediums employed in the past.

It is also an object of this invention to provide an effective method for the continuous coating of a moving electrically-conducting substrate, whether the substrate be a ribbon or a fine wire.

It is another object of this invention to provide a selection of liquid mediums for the conduct of a process whereby a' substantially uniform deposit of boron can be e electrophoretically produced upon asubstrate without experiencing a loss in purity as related to the purity of the starting boron powder.

Four commercial boron powders from different sources 'were'employed in tests and identification of these different 0 boron powders by letter in subsequent tabulated data herein is as follows:

Boron Powder Source "A Manufactured by United States Borax and Chemical Corlaaoratiori and marketed as U.S. Borax 90-92% Amorphous oron. LB Manufactured by American Potash and Chemical Company and marketed as Trona Amorphous Boron-Standard 90-92%. C Manufactured by Hermann C. Starck (Berlin, West Ger- The difliculty posed by this considerable variance from 40 one commercialnboron powder to another and even be-" tween boron powders obtained from different manufacturers and apparently made by the same process, is that very erratic results are obtained, when these different commany) and marketed as l'gnitor Grade. D Manufactured by Cooper Metallurgical Associates and marv keted asCooper Grade A.

Approximate chemical analyses of these materials are reported in Table I below:

TABLE I 'A B C D Boron 90-92 90 92 Present 99135 Crystal Fomn. Amorphous Amorphous .ND

mercial borons are electrophoretically deposited from the same known liquid medium. Thus, with a given liquid medium one commercial boron powder may be deposited at a comparatively rapid rate relative to a second boron powder from another manufacture electrophoretically deposited under exactly the same operating conditions using the same liquid medium. However, once a given liquid medium has been proven to be suitable for a given commercial boron powder, subsequent lots from the same It is known that boron powders A and B are prepared by-the reduction of boric oxide with magnesium and boron powder D is prepared by electrolysis of-a fused salt bath. Electron micrographs show that boron powder D contained relatively large, rounded. particles and that boron powders A and B contained smaller particles both rounded and angular with the particles of powder A bein g smaller than those of powder B.

The electrophoretic deposition of boronspowders from clear particle 'counters has been described in the litera-' ture (Nuclear Sci. Abstrs., 4, 787 [1950], 0. Flint). The medium in which the boron powder was suspended had the following proportions: methyl alcohol (CR), 200

deposition rates over any obtainable with the above described suspension medium formulation. These formulations are: nitromethane (62.3 percent by weight) and isopropanol (about 31.4 percent by weight); isopropanol (CR); and ethanol (either absolute or 95 percent). Also ml.; tannic acid (CR), 2 g.; and collodion solution (5.12 the nitromethane-isopropanol mixture gave almost as percent), 8 ml. The anode, a copper strip, and the good results by the addition thereto of a small amount cathode, a copper foil, were held cm. apart. The boron of the protein zein in the approximate proportions found powder suspended in the medium in each instance useful in applying a coating of a mixture of nickel oxide amounted to 20 g. and in each case was deposited on the 10 and chromium-nickel powder (Modern Electroplating, positive electrode. The results published in the article F. A. Lowenheim, ed., Wiley, New York, 2nd edition, and the results from tests conducted with three commer- 1963, page 720). The formulation was prepared by adding cial boron powders deposited from suspension in the same 1 gm. of the protein zein to 100 milliliters of the mixture formulation are shown in Table II. of nitromethane and isopropanol, mixing and shaking and then allowing to' stand overnight. About'2 milliliters TABLE II of the same nitromethane-isopropanol solution together Ammllhwsy Boron Powder with about 10 grams of boron was added to 98 milliliters 010 microns (Flint) A B D of liquid decanted from the aforementioned nitromethane- Tim isopropanol-zein mixture and used as the suspension e, min- 5 2 20 Temp,, 0,1 38 3 34.355 35-33 5 20 medium for the electrophoretic deposition of boron pow- Current ma. 120 30-24 22- 16-6 PotentialvL 60 6M1 6H1 6M9 ders B and D with the remarkable success displayed in Deposit weight, in 57. 2 50 21 509 Table III (formulation a). Attempts to electrophoretically 2??? area 0111-2 15 12-95 17-43 deposit boron powders A and C under similar conditions MgJwat/t minute 0 1. g 0 1 0 8. 9 3g 2 with formulation a produced no deposit of boron. Boron MgJsec. cm. .0 l 0. Coveragemg/cm, as as L2 559 25 powder B was successfu ly electrophoretically deposited Current density, ma./cm. s 2.1 1.1 1.2 from the mixture of nitromethane and isopropanol (not Watt mmutes 36 24 including the protein zein and so indicated in Table III) Wherearangeisgiven, the first numberisthe initial reading and the and this same boron powder was also successfully elecsecmd number the finalreadmgtrophoretically deposited from isopropanol (C.P.) as

The deposit of boron powder in each instance was relais shown in Table III.

TABLE III Boron Powder B B B B D a less zein isopropano 22 Room Room Room 2. 2 2 2 14-6 j 60 450 450 450 450 Deposit \vt., mg 375 5,605 2, 403 829 2.002 Angde area, cm 15.83 .55 14.23 14.23 13.50

Mg. lwatt min. 356 1335 921 MgJsec. cmJL. 0.32 1. 41 0. 49 1. 23 Coverage, mgJcm. 385 169 68 147 Current density ma./cm. 0. l2 0. l4 0. 07 Watt minutes 0.6 15.75 1.8 0.9

1 Where a range is given, the first number is the initial reading and the second number is the final reading.

2 1 ma. used for calculations. Electrode separation was 8 cm. in all experiments. Reference to Room temperature indicates about 23 C.

tively soft and most of each deposit was easily removed by holding the coated electrode in a stream of water and gently rubbing with the fingers. After removal of most of the deposit in this manner, a very thin, spotty, adherent layer remained. The rate of deposition of boron powder A decreased in value when the deposition was repeated with only the finer particles of boron powder A selected by eleutriation in water. When boron powder A was milled, however, and the electrophoretic deposition was repeated, the deposition rate increased to 0.082 mg./sec./cm. Unfortunately, the deposit obtained was rough, contained a substantial amount of impurities introduced during the milling operation and on standing the deposit cracked and fell ofl? the electrode. Boron powder B did not show any greatly improved rate of deposition after milling in the same manner. An attempt was made to continuously coat boron powder B on tungsten wires having diameters of 1, 2, and 10 mils employing potential up to 300 v., but essentially no deposit was obtained.

In contrast to the relatively low rates of deposition encountered with the above prior art suspension medium at least three other formulations have been discovered each of which has been demonstrated to be capable of enabling the electrophoretic deposition of at least one of the boron powders A, B, C and D at greatly improved A most unusual aspect of the electrophoretic deposition from the fluid mediums described herein is that such deposition proved to be equally successful on both stationary and moving electrodes, whenever the boron powdersuspension medium was proper. For example, continuous coatings have been applied to moving ribbons of tungsten and to l-mil tungsten wires to stationary copper plates and 4-mil diameter filaments of graphite. In the case of the tungsten wire, the wire was conducted through a one liter pot containing a stirred slurry of the boron powder in formulation a, the pot being heated by means of a flexible electric heating tape wrapped therearound with constant monitoring of the temperature of the slurry. The tungsten wire was drawn through the slurry at a rate of about 5 cm./minute, the wire being the anode and the cathode being a inch diameter copper rod. In similar tests employing the prior art suspension medium referred to hereinabove only occasional spots of boron were deposited on the moving wire.

In the case of the graphite filaments, 2-inch lengths of these filaments (about 4 mils in diameter) were coated to a total diameter of 10 mils in 0.08 minutes at 20 volts using the formulation a containing either boron powder B or boron powder D. As an example of the continuous coating of the tungsten wire, 2 mil wire was continuously coated with boron to a total diameter of 10 mils at the aforementioned draw rate of 5 cm./minute. In all cases the deposit of boron was porous and moderately adherent.

Table IV shows that boron powders A, B and C were each successfully electrophoretically deposited using 95 percent ethyl alcohol. Also, shown therein is the effect of the addition of various amounts of water to formulation a and the successful high-rate deposit of boron powder B from suspension in absolute ethyl alcohol.

cally deposited at very rapid rates independently of the method of manufacture of boron powder.

Various modifications are contemplated and may ob-- viously be resorted to those skilled in the art without de parting from the spirit and scope of the invention, as hereinafter defined by the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

TABLE IV Boron Powder B B B B B O C A Formination a a+1 ml. H1O a+4 ml. Hi0 Abs. EtOH 95% EtOH 95% EtOH 95% EtOH 95% EtoH Time, min- 2 2 2 2 2 2 2 2 Temp., C Room Room Room Room Room Room Room Room Current, ma 6 7 1 28-16 3-1 74 10 2 10 Potential, v 450 450 4-50 450 450 450 450 450 Deposit wt., mg..- 2, 201 1,635 none 1, 380 822 2 1, 147 2 700 2 1 02 Anode area, cm. 14. 55 14.55 91 8.79 11. 99 6.6 12 Rate, mg./sec./cm. 1. 26 0. 94 0 0. 83 0. 78 0. 80 0.88 0. 71 Coverage, mg./cm. 151 112 0 99 94 96 106 85 1 Where a range is given, the first number is the initial reading and the second number is the final reading. 2 These borons deposited on the-positive electrode, all other deposits were on the negative electrode.

In the case of boron powder A the material as received would not electrophoretically deposit from the 95 percent ethyl alcohol medium. However, after digesting the boron powder at a near boil for about 1 hour in a solution of hydrochloric acid (1 HCl [36%]/ 4 H O, by volume) the results reported in Table IV were obtained. This digesting procedure eliminates the oxide coating from over the boron particles (as well as other basic impurities such as magnesium and magnesium oxide), the results being in general agreement with the results obtained by the electrophoretic deposition of ground boron particles. The use of grinding to upgrade boron powder by exposing uncoated boron particles has the disadvantage, however, of introducing impurities such as silica or silicates into the boron powder. Other materials may be used in place of the HCl solution for this digestion step, as for example, boiling water as long as the prime object of the elimination of the oxide coating from the boron particles is accomplished.

In all cases the deposits occurring in the electrophoretic deposition of boron powder from the suspension mediums of this invention are uniform in thickness and in content. Similarly, the deposits contain no larger amount of im purities than is present in the starting material. The amount of boron powder placed in suspension in the particular medium employed is not critical except, of course, boron powder must be constantly added to formulations, when conducting continuous electrophoretic deposition of the boron.

Therefore, by the practice of this invention those skilled in the art are provided with a selection of suspension media from which it is relatively certain that properly treated boron powder can be successfully electrophoreti- 1. In the electrophoretic deposition of commercial boron wherein finely divided particles of the boron are suspended in a suitable liquid medium containing a pair of spaced electrodes and a potential difference is applied to the two electrodes, the improvement of employing a suspending liquid medium selected from the group consisting of (a) a mixture consisting essentially of isopropanol and nitromethane, (b) isopropanol, (0) ethanol and (d) a mixture consisting essentially of isopropanol, nitromethane and zein.

2. The improvement substantially as recited in claim 1 wherein the commercial boron powder is subjected to a digestion procedure to remove any oxide coating from the boron powder prior to conducting the electrophoretic deposition.

3. The improvement substantially as recited in claim 1 wherein the deposition is continuous.

4. The improvement substantially as recited in claim 3 wherein the substrate is a wire having a diameter of less than about 10 mils.

5. The improvement substantially as recited in claim 1 wherein the rate of deposition is at least 0.1 mg./sec. cm. whereby a uniform coating over the substrate results.

References Cited UNITED STATES PATENTS 1/1951 Flint 204l81 2/1966 Klach et a1. 204181 

